1. Field of the Invention
The present invention relates to a semiconductor device, and methods of forming and operating the same, and more particularly, to a NAND flash memory device and methods of forming and operating the same.
2. Description of the Related Art
Flash memory devices are non-volatile, maintaining stored data even when its power supply is interrupted. Also, flash memory devices can both write and erase data.
Among the family of flash memory devices, the NAND flash memory device has a plurality of memory cells that share a common source and a common drain, so it is very adaptable to high integration.
A general construction of the NAND flash memory device will now be described with reference to the accompanying drawings.
FIG. 1 is an equivalent circuit diagram of a NAND flash memory device according to the related art.
Referring to FIG. 1, the NAND flash memory device includes a plurality of cell strings arranged in parallel in a row direction. Each of the cell strings includes a plurality of memory cells 10 connected in series. Also, the cell strings include a ground selection transistor 20 connected in series to one end of each of the plurality of memory cells 10, and a string selection transistor 30 connected in series to the other end of each of the plurality of memory cells 10. The plurality of memory cells 10 are arranged in two dimensions, along row and column directions. Although not shown in the drawing, the memory cell 10 has a floating gate as a means of storing data.
Gates of the ground selection transistors 20 arranged in the column direction are connected to a ground selection line ‘GSL’, and gates of the string selection transistors 30 arranged in the column direction are connected to a string selection line ‘SSL’. A plurality of word lines ‘WL’ are arranged between the ground selection lines ‘GSL’ and the string selection lines ‘SSL’. Each of the plurality of word lines ‘WL’ is connected with (control) gates of the memory cells 10 arranged in the column direction. Source regions of the ground selection transistors 20 are connected to a common source line ‘CSL’. The common source line ‘CSL’ is connected with the source regions of the plurality of ground selection transistors 20 arranged in the column direction. Drain regions of the string selection transistors 30 are connected to the bit line ‘BL’. One bit line ‘BL’ is connected to each of the plurality of strings.
A programming operation for a selected cell 10s of the aforementioned related art NAND flash memory device will now be described. When a ground voltage is applied to the common source line ‘CSL’, a constant voltage is applied to the string selection line ‘SSL’, and a ground voltage is applied to the ground selection line ‘GSL’, the ground selection transistor 20 is turned off.
The ground voltage is applied to the selected bit line ‘BL’ connected to the selected cell 10s, and the constant voltage is applied to the non-selected bit lines ‘BL’. A programming voltage is applied to the selected word line ‘WL’ connected to the selected cell 10s. At this time, a pass voltage is applied to the non-selected word lines ‘WL’. The pass voltage is a voltage capable of turning on both the memory cell 10 storing data and the memory cell 10 not storing data. Accordingly, the ground voltage supplied to the selected bit line ‘BL’ is applied to a channel of the selected cell 10s, and the programming voltage applied to the selected word line ‘WL’ is applied to the gate of the selected cell 10s. As a result, electrons in the channel of the selected cell 10s tunnel through a tunnel insulation layer by Fowler Nordheim tunneling, and are then stored in a floating gate.
Meanwhile, a voltage obtained by subtracting a threshold voltage of the string selection transistor 30 from the constant voltage applied to the non-selected bit line ‘BL’ is applied to a channel region of the non-selected cell 10 connected to the selected word line ‘WL’. By doing so, the string selection transistor 30 connected to the non-selected cell 10 is automatically turned off, so that the channel of the non-selected cell 10 is in floating state. As a result, when a high programming voltage is applied to the selected word line ‘WL’, the channel voltage of the non-selected cell 10 is boosted by a capacitor coupling. Accordingly, a voltage difference between the non-selected cell 10 and the selected word line ‘WL’ is decreased, so that the non-selected cell is prevented from being programmed.
However, as the channel voltage of the non-selected cell 10 is boosted, a leakage current may be generated between the drain region and the source region of the ground selection transistor 20 connected to the non-selected cell 10, which will be described with reference to FIG. 2.
FIG. 2 is a cross-sectional view of a NAND flash memory device according to the related art.
Referring to FIGS. 1 and 2, FIG. 2 shows the ground selection transistor 20 of the cell string having the non-selected cell 10 connected with the selected word line ‘WL’ and the memory cell adjacent to the ground selection transistor 20.
A cell gate line 2 and a ground selection gate line are disposed on a semiconductor substrate 1. Cell source/drain regions 4 and 4′ are disposed in the semiconductor substrate 1 at both sides of the cell gate line 2. A common source region 5 is disposed in the semiconductor substrate 1 at one side of the ground selection gate line 3. The cell source/drain region 4′ between the cell gate line 2 and the ground selection gate line 3 is shared by the memory cell 10 and the ground selection transistor 20. In other words, the cell source/drain region 4′ corresponds to the drain region 4′ of the ground selection transistor 20.
The boosted channel voltage of the non-selected cell 10 connected with the selected word line ‘WL’ can be applied to the drain region 4′ of the ground selection transistor 20 via sources/drains of the neighboring memory cells 10. The boosted channel voltage may be considerably higher than the constant voltage. For example, when the programming voltage is about 18 V, the boosted channel voltage is about 8 V, which is considerably high. Accordingly, even when the ground selection transistor 20 is in an Off state, a punch-through phenomenon may be generated between the drain region 4′ and the common source region 5. Also, drain induced barrier lowering (DIBL) phenomenon occurs in the ground selection transistor 20 by the boosted channel voltage, so that a leakage current may be generated between the drain region 4′ and the common source region 5. Unfortunately, the leakage current thus generated decreases the boosted channel voltage, so that the non-selected cell 10 may be inadvertantly programmed.
Also, the high boosted channel voltage may degenerate the punch-through characteristic between the source and drain of the string selection transistor 30. Accordingly, a leakage current may be generated through the string selection transistor 30.
According to the high integration trend of semiconductor devices, line widths of ground selection gate lines 3 are continuously decreasing. To this end, the leakage current between the drain region 4′ and the common source region 5 is becoming more serious.